Download Name Chemistry ___/___/___ Kinetic Molecular Theory

Name ______________________
Chemistry
___/___/___
Kinetic Molecular Theory, Vapor Pressure and Phase Diagrams
The Kinetic Molecular Theory of Gases



The Kinetic Molecular Theory (KMT) is a model that attempts to explain the properties of an ideal gas.
The KMT states:
o The particles of an ideal gas are so small compared with the distances between them that the volume of individual
particles can be assumed to be negligible (zero).
o The particles of an ideal gas are in constant motion. The collisions of the particles with the walls of the container
are the cause of the pressure exerted by the gas.
o The particles of an ideal gas are assumed to exert no forces on each other; no attraction or repulsion between
particles.
o The average kinetic energy of gas particles of an ideal gas is assumed to be directly proportional to the Kelvin
temperature of the gas.
An ideal gas is a hypothetical concept. No gas exactly follows the ideal gas law, although many gases come very close at
low pressures and/or high temperatures. The ideal gas behavior can best be thought of as the behavior approached by real
gases under certain conditions
Vapor Pressure








The water level in a glass of water left out will gradually decrease until all of the
water has evaporated. This vaporization of water molecules occurs as water
molecules gain enough kinetic energy to overcome the attractive forces keeping
them in the liquid. Remember that as one molecule evaporates, the particles left
behind cool. In order to evaporate a particle must absorb energy. Once this
happens, it changes state and the particles left behind have a lower average kinetic
energy.
When a liquid is placed in a sealed container, the amount of liquid at first decreases but eventually becomes constant. The
decrease occurs because there is an initial net transfer of molecules from liquid to the vapor state. This evaporation process
occurs at a constant rate at a given temperature.
The process by which vapor molecules re-form a liquid is called condensation. Eventually, enough vapor molecules are
present above the liquid so that the rate of condensation equals the rate of evaporation. At this point no further net change
occurs in the amount of liquid or vapor because the two processes exactly balance each other; the system is at equilibrium.
This process is highly dynamic.
Liquids with high vapor pressure are said to be volatile. The vapor pressure of
a liquid is principally determined by the size of the intermolecular forces in the
liquid.
In general, substances with large molar masses have relatively low vapor
pressures, mainly because of the large dispersion forces. The more electrons a
substance has, the more polarizable it is, and the greater are the dispersion
forces.
Vapor pressure increases significantly with temperature. The diagram to the
right shows how vapor pressure increases with temperature for 4 different
substances
Like liquids, solids have vapor pressures. Under normal conditions iodine and
dry ice(solid CO2) sublime; they go directly from the solid to the gaseous state
without passing through the liquid state.
Scientists use an instrument called a manometer (pictured below) in order to measure the pressure exerted by a gas. By
comparing the heights of the mercury in the U-tube, scientists can calculate a substance’s vapor pressure. In the diagrams
below, the first graphic shows a higher vapor pressure than the second. As temperature increases, the vapor pressure of a
liquid also increases, so you can assume that the first liquid is at a higher temperature than the second.
The images to the left show two
manometers in which liquid A has
a higher vapor pressure than
liquid B.
Phase Diagrams
A phase diagram is a graphic representation of the relationship between
the physical state of a substance and its pressure and temperature. A
phase diagram describes conditions and events in a closed system. The
phase diagram for water is shown to the right. A line that separates any
two regions gives the conditions at which those two phases exist at
equilibrium. The point at which the three segments meet is called the
triple point. The triple point is the point on a phase diagram where all
three states of a substance are present. For water, the triple point occurs
when the pressure is 0.60 kPa and the temperature is 0.0098 °C. Critical
temperature is defined as the temperature above which vapor cannot be
liquefied no matter what pressure is applied. The critical pressure is the
pressure required to produce liquefaction at the critical temperature.
Together the critical pressure and the critical temperature define the
critical point.
A phase diagram can be used to determine the melting point and boiling
point for a substance at various temperatures and pressures. The normal
melting point of a substance is the temperature at which the solid and
liquid states have the same vapor pressure under conditions at standard pressure (1 atm or 101.3 kPa). For water, the normal melting
point is 0°C. The normal boiling point is the temperature at which the vapor pressure of the liquid is equal to standard pressure (1
atm or 101.3 kPa). For water, the normal boiling point is 100°C. Boiling point is the temperature at which the vapor pressure of a
liquid is just equal to the external pressure. As you can see from the diagram above, water can be made to melt or boil at temperature
other than its normal melting and normal boiling points. Note that the solid/liquid
boundary on the phase diagram for water has a negative slope. This means that the
melting point of ice decreases as the external pressure increases. This behavior,
which is opposite of most substances, occurs because the density of ice is less than
that of the liquid water at its melting point. When water freezes it expands. The
low density of ice means that ice formed on rivers and lakes will float, providing a
layer of insulation that helps prevent bodies of water from freezing solid in the
winter. Aquatic life can therefore continue to live through periods of freezing
temperatures. The phase diagram for carbon dioxide is shown to the right. The
solid/liquid line has a positive slope, since solid CO2 is denser than liquid CO2.
Carbon dioxide is often used in fire extinguishers, where it exists as a liquid at 25°C
under high pressures. Liquid CO2 released from the extinguisher into the
environment at 1 atm immediately changes to a vapor. Being heavier than air, this
vapor smothers the fire by keeping oxygen away from the flame. The liquid/vapor
transition is highly endothermic, so cooling also results, which helps to put out the
fire.
Homework:
1. List the four properties assumed by the kinetic molecular theory.
2. What is vapor pressure?
3. What is meant when a liquid is said to be volatile?
4. Which of the following substances would have the lowest vapor pressure, CH4, C2H6, C3H8 or C4H10? Explain!
5. What happens to vapor pressure as temperature increases?
6. Do solids exert a vapor pressure?
7. Draw a graph that shows the relationship between vapor pressure and temperature.
8. What is a manometer?
9. Equal quantities of different liquids are placed in closed manometers at 20°C. Which liquid has the lowest vapor pressure?
10. Define triple point.
11. What are the triple point values of pressure and temperature for water?
12. Define critical temperature.
13. Define critical pressure.
14. Define critical point.
15. Define normal melting point.
16. Define normal boiling point.
17. Define boiling point.
18. What is the difference between boiling point and normal boiling point? (Don’t just tell me the definitions!)
19. What can be done to get water to boil below its normal
boiling point?
20. What characteristic of water’s triple point graph
indicates that solid water is less dense than liquid water?
21. On the diagram to the right, label all of the following:
normal melting point, normal boiling point, solid, liquid,
gas, triple point, the solid-vapor border, the liquid-vapor
border and the solid-liquid border.